1. Field of the Invention
The present invention relates to a toner for developing electrostatic latent image used upon developing an electrostatic latent image, which is formed by an electrophotographic method or an electrostaic recording method, with a developer, and a process for producing the same, and it also relates to a process for forming an image, an apparatus for forming an image and a toner cartridge, which use the toner for developing electrostatic latent image.
2. Description of the Related Art
A process for visualizing image information through an electrostatic image, such as an electrophotographic process, is being widely applied to various fields. In the electrophotographic process, after uniformly charging a surface of a photoreceptor, an electrostatic image is formed on the surface of the photoreceptor, the electrostatic latent image is visualized as a toner image through development with a developer containing a toner, and the toner image is transferred and fixed to a surface of a recording medium to form an image.
As the developer used herein, a two-component developer containing a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner solely have been known. The toner used in the developers is generally produced by a kneading and pulverizing method, in which a thermoplastic resin is melted and kneaded with a pigment, a charge controlling agent and a releasing agent, such as wax, and after cooling, it is finely pulverized and classified. In the production of the toner, fine particles of an inorganic material and/or an organic material may be added to the surface of the toner particles depending on necessity for improving the flowability and the cleaning property. While the production process of the toner can provide an excellent toner, it involves several problems described below.
The shape and the surface structure of the toner produced by the ordinary kneading and pulverizing method are irregular, and the shape and the surface structure of the toner cannot be intentionally controlled while they are delicately changed by the pulverization property of the materials used and the conditions for the pulverizing step. Furthermore, in the kneading and pulverizing method, there is a limitation in selection of materials used for producing a toner. Specifically, it is necessary that a resin colorant dispersion used as the material is sufficiently brittle and is capable of being finely pulverized by a production apparatus that can be employed under the economical circumstances. However, when the resin colorant dispersion is made brittle to satisfy the demand, there are some cases where further fine powder is formed, and the shape of the toner is changed, by a mechanical shearing force applied in a developing device. Due to the phenomenon, the fine powder is firmly fixed on the surface of the carrier to accelerate deterioration of charge of the developer in the two-component developer. In the one-component, there are some cases where the particle size distribution of the toner is broadened to cause scattering of the toner, and the development property is lowered by the change of the shape of the toner to deteriorate image quality.
In the case where a large amount of a releasing agent, such as wax, is internally added to form a toner, the releasing agent is liable to be exposed on the surface of the toner depending on the combination with the thermoplastic resin. Particularly, in the case where the toner is produced with a combination of a resin with its elasticity increased that is slightly difficult to be pulverized due to a high molecular weight component with brittle wax, such as polyethylene, exposure of the polyethylene is often observed on the surface of the toner. In this case, although it is advantageous to the releasing property on fixing and to cleaning of a non-transferred toner remaining on the surface of a photoreceptor, the polyethylene exposed on the surface of the toner easily migrates to other members with a mechanical force, whereby the developing roll, the photoreceptor and the carrier are liable to be contaminated to bring about decrease in reliability.
Furthermore, there are cases where a flowability assistant is added to suppress decrease in flowability due to the irregular shape of the toner. In this case, however, there are some cases where sufficient flowability of the toner cannot be obtained, and the fine particles of the flowability agent added to the sure of the toner migrate to concave parts on the toner with a mechanical shearing force upon forming an image to lower the flowability with a lapse of time and to bury the flowability agent into the toner, whereby the development property, the transfer property and the cleaning property are deteriorated. The image quality is liable to be lowered when a toner recovered by cleaning is returned to the developing device for reusing. In the case where the amount of the flowability agent added to the surface of the toner is increased in order to avoid the problem, black spots are formed on a photoreceptor, and the fine particles of the flowability agent are scattered.
In recent years, a process for preparing a toner by an emulsion polymerization and aggregation method is proposed as a method enabling intentional control of a shape and a surface structure of a toner (as described, for example, in JP-A-63-282752 and JP-A-6-250439). In the production process of a toner, generally, a resin fine particle dispersion produced by emulsion polymerization and a colorant particle dispersion produced by dispersing a colorant in a solvent are at least mixed to form aggregates having a diameter corresponding to a particle diameter of a toner, and the aggregates are coalesced by heating to form a toner. The production process of a toner not only realizes decrease in particle diameter of the toner, but also such a toner can be obtained that is considerably excellent in particle size distribution.
Furthermore, in recent years, there is remarkable tendency to decrease a diameter of a toner for realizing a high-definition image upon forming a color image associated with an increasing demand for high image quality. However, in the case where the diameter of the toner is simply decreased with maintaining the conventional particle size distribution, the problems caused by contamination of a carrier and a photoreceptor and scattering of the toner caused by the presence of a toner fraction on the small diameter side in the particle size distribution become serious, and therefore, it is difficult that the high image quality and the high reliability are simultaneously realized. Accordingly, it is also necessary that the particle size distribution is narrowed, and simultaneously, the particle diameter is decreased. The production process of a toner utilizing the emulsion polymerization and aggregation method is advantageous from this standpoint.
A toner is being required, in recent years, to have a low temperature fixing property to attain a high-speed operation and energy saving, which are demanded by the use of digital equipments and improvement in productivity of office documents. From the point of view, a toner produced by the emulsion polymerization and aggregation method has excellent characteristics in low temperature fixing property owing to the narrow particle size distribution and the small particle diameter.
In order to assure the releasing property upon flying, in addition to the low temperature fixing property, a surface of a member in contact with a toner image, such as a fixing roll, is coated with a fluorine resin film, such as polytetrafluoroethylene, to decrease the surface energy thereof.
However, in the case where the surface of the fixing roll is heated with a heat source incorporated in the fixing roll, there are some cases where effective thermal conduction from the heat source to the surface of the fixing roll is impaired by the fluorine resin film. Therefore, there is a limitation of the thickness of the fluorine resin film provided on the surface of the fixing roll. In the case where the thickness of the fluorine resin film is decreased to accomplish effective thermal conduction, the low wetting property on the surface of the fixing roll cannot be maintained for a long period of time due to wear of the fluorine resin film. Accordingly, development of such a toner is demanded that enables avoidance of coating of a fluorine resin film having low surface energy on a surface of a member in contact with a toner image, such as a fixing roll.